Implantable medical devices (IMDs) for use in cardiac pacing and defibrillation are well known. Most of these devices include sense amplifier circuitry for detecting intrinsic cardiac electrical activity so that the devices may be inhibited from generating unnecessary stimulating pulses when a heart is functioning properly. While the present invention is not limited to any one IMD, it will, for the sake of brevity, be described with respect mainly to pacemaker-cardioverter-defibrillators (PCDs).
Dual-chamber cardiac pacemakers typically have separate sense amplifiers and associated circuitry for atrial and ventricular sensing. The sense amplifiers detect the presence of intrinsic signals, that is P-waves occurring naturally in the atrium and R-waves occurring naturally in the ventricle. Upon detecting an intrinsic signal, sense amplifier circuitry generates a signal for output to other components which then inhibits the delivery of a pacing pulse to the corresponding chamber.
It is desirable to accurately and reliably measure the response of the heart to an electrical stimulation pulse. Measuring such a response permits, among others, the determination of a patient's stimulation threshold, or the minimum energy a stimulating pulse must contain for a cardiac response to be evoked. Once a patient's stimulation threshold is determined, the energy content of stimulating pulses may be adjusted to avoid delivering pulses having unnecessarily high energy content. Minimizing the energy content of stimulating pulses is believed to have physiological benefits, and additionally reduces power consumption, a key concern in the context of battery-powered IMDs.
such capture detection and management is also useful in controlling a pacemaker's pacing rate, for ascertaining the physiological effect of drugs, or for diagnosing abnormal cardiac conditions. As used herein, the term “capture” means the successful evocation of a stimulated response in cardiac tissue by a pacing pulse. Capture is discussed in detail, for example, in U.S. Pat. No. 5,601,615 to Markowitz et al. and U.S. Pat. No. 6,163,724 to Hemming et al. “Loss of Capture” (or LOC) as used herein, indicates the failure to produce an evoked response.
Immediately following delivery of a pacing pulse to cardiac tissue, a residual post-pace polarization signal (or polarization potential) is produced by the charge induced in the tissue by the delivery of the pacing pulse. If the pacing pulse causes an evoked response in the cardiac tissue, then an evoked response signal is superimposed atop the typically larger amplitude polarization potential. As a result, conventional pacemakers either cannot differentiate, or have difficulty differentiating, between post-pacing pulse polarization potential and evoked response potential.
This problem is further complicated by the fact that residual polarization potentials typically have high amplitudes, even when an evoked response signal occurs. Consequently, it becomes difficult to detect an evoked response potential using a conventional pacemaker sense amplifier employing linear frequency filtering techniques.
Some pacemakers employ sensing and timing circuits that do not even attempt to detect evoked response potential until the polarization potential is no longer present or has subsided to some minimal amplitude level. With respect to capture detection, however, such sensing after the polarization potential is no longer present typically occurs a significant period of time after any evoked response signal has occurred. As a result, these pacemakers cannot reliably detect evoked response signals. Thus, a need exists for reliably determining whether or not an evoked response has occurred in a pacing environment.
Polarization signals typically arise due to the tissue-electrode interface storing energy after a pacing stimulus has been delivered. There are typically two tissue-electrode interfaces in a pacing circuit: one for the tip electrode, and one for the ring (or PCD housing) electrode. The stored energy dissipates after the pacing pulse is delivered, creating the subsequent polarization potential.
Another problem with capture management is peculiar in multi-site pacing, e.g., bi-ventricular pacing. In single ventricular pacing, the ventricular threshold may be determined by incrementally decreasing the pacemaker output until loss of capture is detected, either by human operator or by the pacemaker logic in the capture management function. This task is simplified by the fact that loss of ventricular response is easily detected by lack of an evoked QRS complex or wave following the pacing pulse, e.g., there is no cardiac contraction in response to the pacing pulse.
In bi-ventricular pacing, the task of the threshold measurement is somewhat more complicated. For instance, in most patients, the left ventricular threshold is higher than the right ventricular threshold (this holds true in spite of recent technological improvements in the design and configuration of left ventricular leads). Yet, the right ventricular pacing electrode typically has ideal contact with the endocardium. Therefore, it is believed to be more effective than electrodes located within the coronary vein that deliver the pacing pulse to the epicardium.
If bi-ventricular threshold detection is done in a manner similar to that described above with respect to single ventricle pacing, the incremental decrease of the left ventricular output will cause loss of capture (LOC) of the left ventricle as soon as the output falls below the left ventricular threshold. However, this will not be manifested in loss of the QRS complex (loss of the ventricular contraction) following the pacing pulse because the right ventricular pacing pulse continues to pace. That is, the depolarization wave will still occur, spreading from the right ventricle electrode implantation site, e.g., the apex of the right ventricle. In other words, while the evoked QRS complex morphology may change, it will still be present, indicating capture was detected.
Similarly, incrementally decreasing the right ventricular output will cause right ventricular LOC as soon as the output falls below the right ventricular threshold. Once again, this loss of capture will be not manifested in loss of the QRS complex following the pacing pulse because the left ventricular pacing pulse will continue to pace, producing a depolarization wave starting from the left ventricle electrode implantation site. As a result, the depolarization wave propagation will still be present. Thus, bi-ventricular capture management requires more that merely detecting the absence of the QRS complex.
The problems described herein above, e.g., threshold measurement and capture detection in view of polarization potential, may also present themselves in capture management for atrial pacing, e.g., in three and four chamber pacemakers.
Various methods have been proposed in the prior art for improving the ability to detect and measure evoked responses as well as improving other aspects of IMDs.
For example, U.S. Pat. No. 5,312,441 to Mader et al., discloses, in one embodiment, a method and apparatus for discriminating between ventricular depolarizations resulting from normal and abnormal propagation of the depolarization wave front through the ventricles by means of a measurement of a width of the sensed R-wave associated with the depolarization.
U.S. Pat. No. 5,797,967 to KenKnight, proposes an electrical therapy applied to a selected region of selected cardiac tissue, comprising the combination of two discrete therapies: pacing level therapy applied to a localized portion of a region of the selected cardiac tissue having relatively low susceptibility to defibrillation-level shock field strengths; followed by (or occurring simultaneously with) defibrillation therapy applied to portions of the tissue having regions of fibrillating myocardium over which the sub-defibrillation level shocks exert control.
U.S. Pat. No. 5,411,524 to Rahul describes a method and apparatus for synchronization of atrial defibrillation pulses. In one embodiment, a method and apparatus for controlling the timing of delivery of atrial cardioversion or defibrillation pulses is disclosed. In order to determine an appropriate time for delivery of a cardioversion pulse, the method and apparatus of the Rahul patent first determines the average V—V interval associated with the ventricular rhythm in the presence of atrial fibrillation. Based upon this average interval, the apparatus calculates a shorter, derived escape interval, which is used to control timing of delivery of the atrial cardioversion or defibrillation pulse.
U.S. Pat. No. 5,548,868 to Salo et al. proposes a cardiac stimulating apparatus and method. In particular, Salo et al. describes a cardiac defibrillator in combination with a pacing device with adjustable control of the AV interval.
U.S. Pat. No. 6,148,234 to Struble describes a dual site pacing system with automatic pulse output adjustment. In one embodiment, a dual site pacing system is provided with the capability of automatically detecting when there is LOC in one chamber, either ventricular or atrial. Following each delivered pair of pacing pulses to the dual sites, the pacemaker times out an appropriate blanking interval, and then times out a refractory period to coincide with the heart chamber's normal period of refractoriness following a contraction. During the refractory interval, or refractory period, the pacemaker looks to see if an excitation signal is sensed. If so, this means that a chamber was not captured, and the excitation from the other chamber (which was captured by a delivered pulse) had been conducted to the non-captured chamber.
Accordingly, various implementations of systems for capture management in multi-site pacing are known. These systems are described above and in the documents listed in Table I below.
TABLE IU.S. Pa. No.InventorIssue Date6,163,724Hemming et al.Dec. 19, 20006,148,234StrubleNov. 14, 20005,797,967KenKnightAug. 25, 19985,601,615Markowitz et al.Feb. 11, 19975,584,868Salo et al.Dec. 17, 19965,411,524RahulMay 2, 19955,312,441Mader et al.May 17, 1994
All documents listed in Table I herein above are hereby incorporated by reference in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, Detailed Description of the Embodiments, and claims set forth below, many of the devices and methods disclosed in the documents of Table I and others documents incorporated by reference herein may be modified advantageously by using the teachings of the present invention. However, the listing of any such document in Table I, or elsewhere herein, is by no means an indication that such documents are prior art to the present invention.